1. Field of the Invention
The present invention relates to systems and techniques for determining the disposition of potential defects in photo-masks for use in photo-lithography. More specifically, the present invention relates to systems and techniques for extending the field of view of mask-inspection images when determining the disposition of potential defects in photo-masks.
2. Related Art
Photo-lithography is a widely used technology for producing integrated circuits. In this technique, a light source illuminates a photo-mask. The resulting spatially varying light pattern is projected onto a photoresist layer on a semiconductor wafer by an optical system (referred to as an exposure tool). By developing the 3-dimensional pattern produced in this photoresist layer, a layer in the integrated circuit is created. Furthermore, because there are often multiple layers in a typical integrated circuit, these operations may be repeated using several photo-masks to produce a product wafer.
Unfortunately, as dimensions in integrated circuits steadily become a smaller fraction of the wavelength of the light used to expose images of the photo-mask onto the wafer, the structures in or on the ideal photo-mask (also referred to as the target mask pattern) and/or the physical structures in or on the actual photo-mask bear less and less resemblance to the desired or target wafer pattern on the wafer. These differences between the mask pattern and the target wafer pattern are used to compensate for the diffraction and proximity effects that occur when light is transmitted through the optics of the exposure tool and is converted into the 3-dimensional pattern in the photoresist.
From a photo-mask or reticle manufacturing standpoint, the increasing dissimilarity between the photo-mask and the corresponding wafer patterns creates a broad new class of problems in photo-mask inspection and qualification. For example, if a defect in a photo-mask is detected, it is often unclear what impact this defect will have on the final pattern in the photoresist. In addition, photo-mask inspection devices often have a different numerical aperture, different illumination configuration, different field of view (FOV), and even different light wavelength(s) than those used in the wafer exposure tool. As a consequence, the image measured by a photo-mask inspection tool is often neither a perfect replica of the physical photo-mask nor the mask pattern that will be exposed onto the wafer.
One existing approach to the former challenge uses a computer to simulate the resulting wafer pattern based on the inspection images of the photo-mask. By comparing simulations of wafer patterns corresponding to the ideal photo-mask (i.e., the target mask pattern) and an estimate of the actual photo-mask corresponding to the image of the photo-mask, the significance of the defect may be determined. However, because the image of the photo-mask may not be an accurate representation of the actual photo-mask, errors may be introduced when simulating wafer patterns, and thus, when trying to identify or classify defects. For example, the FOV in many mask-inspection systems is often too small. Consequently, when a resulting inspection image is subsequently used in simulations of a photo-lithographic process, the simulation accuracy can be degraded by optical proximity effects associated with the boundary of the inspection image. This may further complicate photo-mask inspection and qualification.
Similar issues arise when inspecting the patterned wafers. Hence, what is needed are photo-mask and patterned wafer inspection techniques that overcome the problems listed above.